1. Field
The present disclosure describes chip resistors and other resistive devices with improved power handling capability while maintaining low capacitance essential for circuits to operate efficiently in the microwave region of the radio frequency (RF) spectrum.
2. Description of the Related Art
Chip resistors are a common element in RF and Microwave circuits. The various uses for chip resistors generally include one or more of the following: change the amplitude of an RF signal, limit the current in an RF transmission line, or alter the impedance of the transmission line.
As circuit board sizes decrease, designers are faced with the challenge of fitting or positioning more and more circuitry into smaller spaces. Also, as frequencies increase, designers are trending toward smaller and smaller components to minimize parasitic affects which are detrimental to the functioning of circuits. However, decreasing board sizes and smaller components present challenges to heat dissipation in order to prevent premature failure.
Physically small devices tend to have better performance in the microwave region of the RF spectrum because they constitute a small fraction of a wavelength at the frequencies of interest. This reduces detrimental parasitic effects due to excessive inductance and capacitance. As the size of the component decreases, the power handling capability also decreases.
Heat dissipation can be improved by using advanced materials having good thermal conductivity, such as Diamond, Beryllium Oxide, or Aluminum Nitride. Because these materials are either very expensive or toxic, they fail to provide a viable commercial solution. In addition, when using a higher thermally conductive material, the heat generated in a conventional surface mount chip resistor is dissipated through the circuit traces or through the air, which fail to provide satisfactory thermal paths.
Therefore, there exists a need for a chip resistor that combines small physical size, high power handling capability, low capacitance, and low cost. For example, a typical, commercially available, surface mount resistor, in the 0402 size, measures 0.040×0.020 inches (1.0×0.5 mm) and has a power rating of about 0.063 watts. This is insufficient for many applications. If higher power is forced through the surface mount resistor, it will heat to the point that the solder connections will melt leading to a catastrophic failure of the circuit.
Using a physically larger surface mount resistor with a higher power rating can overcome these drawbacks. However, because current design trends favor smaller circuit boards, circuit boards may lack the physical space for the inclusion of a larger surface mount resistor. Even if sufficient space was available, a larger surface mount resistor will have more capacitance and/or inductance, requiring more elaborate tuning measures to minimize these effects. Circuitry necessary to tune out these parasitics takes up valuable board space and reduces the bandwidth of the circuit, an unsatisfactory side effect. In addition, many boards are already designed for optimal RF performance and the dimensions of the circuit traces cannot be changed without a substantial performance, financial, and delivery impact. Designers must use the layout as it is already manufactured.
There is a need for a chip resistor which matches the current industry-standard pad size and configuration, which drastically improves the power handling capability, and at the same time, maintains the low capacitance essential for circuits to operate efficiently in the microwave region of the RF spectrum.